Cassian taught Jyn to be selfless and to take action in her life and Jyn taught Cassian that doing the right thing sometimes means defying authority. They challenged eachother and became better because of it. They saw hope in one another and it carried them till the very end.
What do you think of Lucien after ACOMAF? I know people are pissed that he didn't stick up for Feyre, but at the same time, in the past he's always gotten screwed over whenever he's defied authority.